bpo-31764: Prevent a crash in sqlite3.Cursor.close() in case the Cursor object is uninitialized (GH-3958) (#4303)

(cherry picked from commit edb13ae48c)

Modules/_sqlite/cursor.c

@@ -916,6 +916,11 @@ PyObject* pysqlite_noop(pysqlite_Connection* self, PyObject* args)
 
 PyObject* pysqlite_cursor_close(pysqlite_Cursor* self, PyObject* args)
 {
+    if (!self->connection) {
+        PyErr_SetString(pysqlite_ProgrammingError,
+                        "Base Cursor.__init__ not called.");
+        return NULL;
+    }
     if (!pysqlite_check_thread(self->connection) || !pysqlite_check_connection(self->connection)) {
         return NULL;
     }